Needles and syringes have posed a variety of problems for patients and medical personnel who administer agents to the patients, including injection safety, needle stick injury, disposal problems, transmission of blood borne diseases, and needle shortages during mass vaccination campaigns. The replacement of needles and syringes as the primary delivery vehicle for agents has the potential for tremendous cost savings, increased safety and reduction of biomedical wastes.
Aerosol delivery of agents avoids many of the foregoing drawbacks of injection. Much of the equipment used for aerosol delivery is cumbersome and has not been widely employed for many treatment methods. Nebulizers are commonly used in hospitals for aerosol delivery of agents in the treatment of respiratory diseases. In practice, a nebulizer uses compressed gases to convert a solution of the agent into fine droplets. The droplets are administered to the patient through an air stream that the patient breathes inwardly through a mouthpiece or mask. As the patient breathes, the agent is delivered to the patient's lungs and absorbed therein.
Typically, nebulizers rely upon an external compressed gas source to convert a solution of the agent into fine droplets. As a result of the need for an external source of compressed gas, nebulizers tend to be bulky and difficult to move. Further, the effectiveness of a nebulizer depends upon proper inhalation by the patient, which can be difficult to monitor and to teach to the patient.
Currently used jet nebulizers function in the same general way. Liquid is drawn up to an air nozzle by capillary forces and/or the Bernoulli effect. At the nozzle, a high-speed air jet shatters the liquid into droplets. Droplets blast against an impactor to break them up further into smaller droplets. Like most atomization processes, this droplet generation process results in a size distribution. To obtain the desired small aerosol droplets, baffles capture large droplets (which cannot follow the airflow path well), leaving the fine aerosol in the output stream of the nebulizer. The larger droplets recycle to the liquid reservoir of the nebulizer.
This nebulization process is inherently inefficient. Measurements show that typical nebulizers only convert a few percent of the aspirated liquid to fine aerosol droplets. Thus, liquid will normally be recycled well in excess of twenty times before it reaches the desired size and is exhausted from the nebulizer. The inefficiency of the jet nebulizer poses problems to its use for aerosol vaccination. High velocity is needed in the air jet to provide the energy required to break the liquid into sufficiently small droplets, necessitating relatively high air supply pressures in flow rates. Compressing air to provide this supply requires significant power, either human or electric.
Fluid recycling in the nebulizer in the small amount of vaccine required for each dose results in the inability to operate on a dose-by-dose basis. Many doses need to be present in the nebulizer in order for droplet coalescence on the baffles in other surfaces to return liquid to the reservoir. In addition, the repeated mechanical stress of atomization on the vaccination particles in the liquid risks diminishing the viability of the vaccine.
Another drawback of conventional nebulizers is that the components that come in contact with the agent being dispensed must be thoroughly cleaned after each session of use to prevent the growth of bacteria or other contaminants. Such cleaning and maintenance requirements pose a modest challenge in modern medical settings, but can prove to be extremely difficult to achieve with untrained personnel or in underdeveloped regions of the world. Hence, conventional nebulizers are impractical for use in mass vaccination campaigns, especially in underdeveloped countries.
Existing vibrating mesh nebulizers have similar drawbacks. Vibrating mesh devices typically operate by ejecting droplets through tiny orifices of a thin plate (the “mesh”) that is vibrated ultrasonically by an actuator. Existing vibrating mesh devices place the agent to be aerosolized in direct contact not only with the mesh, but also with the actuator. In such devices, the mesh, actuator surfaces, and the fluid pathway in the device are intended for long-term single-patient use and must be cleaned after each use. Cleaning of these devices under field conditions and their use in multi-patient settings, such as in mass vaccination campaigns impose substantial difficulties and costs.
Monitoring or verifying the dose of aerosol delivered to a patient also poses a concern in the administration of aerosols (e.g., aerosol vaccination), especially when young children are involved. Unlike injection, where the delivery of a dose can be clearly observed, the delivery of an aerosolized agent via a nebulizer is more difficult to monitor.
Thus, a need exists for effective systems and methods for administering an agent in an aerosol form, without a needle, and in more accurate dosages. Further, a need exists for delivery systems that are easier to use and maintain and reduce the likelihood of contamination, especially for use in mass vaccination campaigns.